Are there certain things I can look for when buying a new external hard drive or flash drives to ensure that the one I purchase does not contain BadUSB? Would popular commercial brands, such as Seagate, be a safe choice?
Popular commercial brands are not going to have the BadUSB exploit "installed" when it comes from the factory.
You must understand how the badUSB exploit works. BadUSB is an exploit that can be executed against almost ANY USB drive. It's not something that can be exploited on some vendor drives and not other drives.
For your drive to become compromised, one of three scenarios would need to happen:
Vendor gets compromised and unknowingly starts sending out USB devices with firmware malware (BadUSB) installed.
Someone gets their hands on your USB drive and compromises the firmware without your knowledge.
Malware on your PC infects your USB device's firmware.
The chance of any of these scenarios happening are honestly quite small.
A quite detailed blog article by Sophos security on this topic can be found here: https://nakedsecurity.sophos.com/2014/10/06/badusb-now-with-do-it-yourself-instructions/